An exhaust gas purification device comprising a catalyst in an exhaust-gas flow path is known as a device to purify exhaust gases exhausted from an internal combustion engine. This type of exhaust gas purification device is usually provided with a catalyst having an outer diameter larger than the inner diameter of the exhaust-gas flow path in order to improve effect of exhaust gas purification by the catalyst. Thereby, a diameter-expansion flow path is disposed upstream of the catalyst to gradually expand the inner diameter of the exhaust-gas flow path.
However, in a structure comprising such a diameter-expansion flow path, a flow-rate of the exhaust gases in the diameter-expansion flow path is lower around the outer perimeter than in the central part, and thus a bias is caused in the flow-rate distribution of the exhaust gases that flow into the catalyst. Thus, a structure comprising a metal net body in the upstream side of the diameter-expansion flow path is suggested; the metal net body is for guiding the flow-rate distribution of the exhaust gases to uniformity (Patent Document 1).